Acoustic detector

ABSTRACT

An acoustic detector includes a cylindrical support member and a plurality of receiver elements that are disposed on a surface of the cylindrical support member. The plurality of receiver elements are configured to detect acoustic waves in a plurality of azimuthal angular directions.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority to U.S.Provisional Application No. 61/691,602, filed on Aug. 21, 2012, theentire content of which is incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with Government support under CooperativeResearch and Development Agreement (CRADA) Contract NumberDE-AC52-06NA25396 awarded by the United States Department of Energy. TheGovernment may have certain rights in this invention.

FIELD

The present invention relates generally to acoustic interrogation ofrock formations around a borehole, and more particularly to using thecombination of an acoustic source including a single or an array oftransducers in the wellbore coupled to a linear or non-linear materialfor producing an acoustic beam as a probing tool from a borehole tointerrogate the properties of rock formations and materials surroundingthe borehole.

BACKGROUND

Acoustic interrogation of subsurface features tends to be limited by thefrequency bandwidth of practical sources. High frequency signals have arelatively short penetration distance, while low frequency signals donot have collimation and generate unwanted signals within the well bore.It is difficult to generate a collimated acoustic beam signal in thesonic frequency range between about 15 kHz and about 120 kHz from theborehole to probe the rock formation surrounding a borehole withconventional transducers. Conventional sonic acoustic sources have largebeam spread, such that as the frequency decreases, the beam spreadincreases. The beam spread also depends on the diameter of thetransducer, which is limited by the borehole dimension. Sharpdirectivity steering for a particular frequency requires a number ofconditions to be satisfied, including a long source array, uniformcoupling of all the transducers to the rock formation around theborehole and knowledge of the acoustic velocities of the rock formation.In the borehole environment, these conditions are not often achievablebecause of underlying physics constraints, engineering feasibility oroperating conditions, especially when the source signal has broadfrequency bandwidth.

Traditional monopole and dipole borehole acoustic logs have been used tomeasure sonic velocity near the borehole using frequency range less thanabout 8 kHz. However, at this relatively low frequency, azimuthalresolution is relatively low. There are a number of patents thatattempted to overcome this deficiency by using additional receivers todetect the direction of the signals returning to the receivers (see, forexample, U.S. Pat. No. 5,544,127 and references cited within)).Applications for borehole sonic for reflection imaging, refractionimaging, fractures detection and permeability determination have alsobeen proposed (see, for example, U.S. Pat. No. 5,081,611, U.S. Pat. No.4,831,600, U.S. Pat. No. 4,817,059, and U.S. Pat. No. 4,797,859). All ofthese conventional techniques have operational and azimuthal resolutiondeficiency as the source lacks or has insufficient azimuthal directivityand desired frequency bandwidth.

For cement evaluation, ultrasonic waves in the frequency range ofhundreds of kilohertz (e.g., low ultrasonic frequency range between 80kHz and about 120 kHz and ultrasonic frequency range around about 200kHz) have been used to detect a cement gap behind the casing. Eventhough frequencies around 200 kHz allow for good azimuth resolution, thedistance range for detection at around this frequency is very limited,i.e., the depth of penetration to investigate behind the formation andchannels between cement and rock formation is limited for ultrasonicsource at frequency around 200 kHz. Conventional cement evaluation logsuse a frequency of 30 kHz and can investigate deeper. However, theseconventional cement evaluation logs lack azimuthal resolution becausethe wavelength is around the borehole radius and, consequently, theborehole modes would excite the entire borehole. As a result it isdifficult to extract detailed azimuthal information of the cementbonding. In order to overcome this deficiency, multiple sources(emitting in the frequency range between 70 kHz and 120 kHz) andmultiple receivers are used in a Sector Bond Tool (SBT) system. However,even with the use of multiple sources and multiple receivers, theconventional SBT system was not able to cure the deficiencies of theprior conventional cement evaluation logs as the source still lackedazimuthal directivity to effectively detect the existence of smallchannels between the cement and the rock formation.

SUMMARY

An aspect of the present invention is to provide a method forinvestigating cement bonding or rock formation structure near aborehole. The method includes generating an acoustic wave by an acousticsource; directing at one or more inclination and azimuthal angles theacoustic wave towards a target location in a vicinity of a borehole;receiving at one or more receivers an acoustic signal, the acousticsignal originating from a reflection or a refraction or surface wavepropagation of the acoustic wave by a material at the desired location;and analyzing the received acoustic signal to characterize features ofthe material around the borehole.

Another aspect of the present invention is to provide a system forinvestigating cement bonding or rock formation structure near aborehole. The system includes an acoustic source configured to generatean acoustic wave and to direct the acoustic wave at one or moreazimuthal angles towards a desired location in a vicinity of a borehole.The system also includes one or more receivers configured to receive anacoustic signal, the acoustic signal originating from a reflection or arefraction or surface wave propagation of the acoustic wave by amaterial at the desired location. The system also includes a processorconfigured to perform data processing on the received signal to analyzethe received acoustic signal to characterize features of the materialaround the borehole.

Yet another aspect of the present invention is to provide an acousticsource for generating an acoustic beam. The acoustic source includes ahousing; a plurality of spaced apart piezo-electric layers disposed withthe housing; and a non-linear medium filling between the plurality oflayers. Each of the plurality of piezoelectric layers is configured togenerate an acoustic wave when excited with an electrical signal. Thenon-linear medium and the plurality of piezo-electric material layershave an acoustic matching impedance so as to enhance a transmission ofthe acoustic wave generated by each of plurality of layers through theremaining plurality of layers.

Another aspect of the present invention is to provide an acousticdetector that includes a cylindrical support member and a plurality ofreceiver elements that are disposed on a surface of the cylindricalsupport member. The plurality of receiver elements are configured todetect acoustic waves in a plurality of azimuthal angular directions.

These and other objects, features, and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious Figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention. As usedin the specification and in the claims, the singular form of “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a schematic diagram of a system for generating acollimated acoustic beam for characterizing formations and/or materialsnear a borehole, according to an embodiment of the present invention;

FIGS. 1C and 1D show a schematic diagram of an end-fire array ofpolyvinylidene difluoride (PVDF) film acoustic source used forgenerating a collimated acoustic beam, according to an embodiment of thepresent invention;

FIGS. 1E and 1F depict the signal output by the end-fire array of PVDFfilm acoustic source without applying a delaying to an excitationelectrical signal and when applying an appropriate delay to theexcitation electrical signal;

FIGS. 2A-2C are schematic representations of a receiver, according tovarious embodiments of the present invention;

FIG. 3 is schematic diagram of an acoustic measurement system, accordingto an embodiment of the present invention;

FIG. 4A illustrates a characteristic of a parametric array beam pulsesignal emitted by an acoustic source, according to an embodiment of thepresent invention;

FIG. 4B is a fast Fourier transform (FFT) of the acoustic beam signal ofFIG. 4A to obtain the signal in the frequency domain;

FIG. 5 depicts data collected as a function of propagation time andazimuthal angle, according to an embodiment of the present invention;

FIG. 6 depicts a schematic diagram of an experimental set-up withreceiver having a linear array of receiver elements disposed on asurface of a cylindrical configuration, according to an embodiment ofthe present invention;

FIG. 7 depicts reflection data obtained in an experiment similar to thedata shown in FIG. 5 but after performing signal processing to filterout the linear arrivals; according to another embodiment of the presentinvention;

FIG. 8 depicts data collected as a function of propagation time andreceiver number after performing signal processing to filter out thelinear arrivals, according to an embodiment of the present invention;

FIG. 9 depicts another experiment in which the orientation of thereceiver 24 is fixed (i.e., the receiver is not rotated) and the mirroris rotated azimuthally; according to another embodiment of the presentinvention;

FIGS. 10A and 10B depict an experimental acoustic setup, according toanother embodiment of the present invention where FIG. 10A is alongitudinal schematic view of the experimental setup and FIG. 10B is atop view of the experimental setup;

FIGS. 11A-11C show plots of the measured data for various azimuthalorientations or angles, respectively, at about 320 deg., at about 90deg. and at about 165 deg., according to an embodiment of the presentinvention;

FIGS. 12A-12C show plots of synthetic wave forms of acoustic measurementin the frequency range of 15-120 kHz for various borehole conditions,according to embodiments of the present invention;

FIGS. 13A-13C shows simulated frequency chirp propagation data alongwith the time-frequency analysis of the same data, according toembodiments of the present invention;

FIG. 14A depicts the acoustic measurement system disposed within aborehole, according to embodiment of the present invention;

FIG. 14B depicts the acoustic measurement system disposed within aborehole, according to another embodiment of the present invention;

FIG. 14C depicts the acoustic measurement system disposed within aborehole, according to yet another embodiment of the present invention;and

FIG. 15 is a schematic diagram representing a computer system forimplementing the method, according to an embodiment of the presentinvention.

DETAILED DESCRIPTION

FIGS. 1A and 1B is a schematic diagram of a system for generating acollimated acoustic beam for characterizing formations and/or materialsnear a borehole, according to an embodiment of the present invention.The system 10 includes one or more electrical signal generators 12configured to generate signals at a first frequency and a secondfrequency. The signals are transmitted to a signal amplifier oramplifiers 14 that are configured to increase the power of the signals.The signals modified by the amplifier 14 are transmitted to one or moretransducers 16 that are configured to generate acoustic waves at thefirst and the second frequency. The acoustic waves are transmitted to anon-linear material 17, which mixes the waves at the first frequency andthe second frequency by way of wave mixing process to produce acollimated acoustic beam 18 at a third frequency. In one embodiment, thecollimated acoustic beam 18 can have a frequency in the range betweenabout 15 kHz and about 120 kHz. This frequency range can be increased byusing, for example, different transducers and primary frequencies. Thecollimated acoustic beam 18 can be a continuous acoustic signal or mayalso comprise one or more acoustic pulses (e.g., a train of acousticpulses).

The non-linear material 17 can be a liquid, a mixture of liquids, asolid, a granular material embedded in a solid casing, embeddedmicrospheres, acoustic meta-materials, or an emulsion. By way of anon-limiting example of such a non-linear material is Fluorinert FC-43.Fluorinert is selected for its relatively low sound velocity (646 m/s)and high acoustic nonlinearity (β˜7.6). Depending on the operatingconditions in the borehole, other non-linear materials can be used as anon-linear mixing medium with suitable low sound velocity, highnon-linear coupling, absorption length, shock wave length, temperatureand pressure operating ranges, as well as, other requirements requiredby operability specifications. Moreover, the length of the non-linearmaterial can be very compact and can range from between 5 cm to 2 metersfor the frequency range between approximately 15 kHz and approximately120 kHz depending on the type of materials being used. The non-linearmaterial can be disposed in a housing, such as for example a cylindricalcontainer. The axis of the non-linear material-filled housing can bealigned with a borehole axis, such that the difference frequencyacoustic beam that is output by the non-linear material propagates alongthis axis.

The non-linear behavior may be characterized through the analysis of theproperties of P-waves resulting from the non-linear mixing phenomenon inwhich two incident waves at two different frequencies, f₁ and f₂, mix togenerate third frequency components at the harmonics andinter-modulation frequencies f₂−f₁, f₂+f₁, 2f₁ and 2f₂, etc. In anaspect of the invention, the non-linear collinear mixing phenomenon isdesigned to occur in the non-linear material inside the wellbore. Ingeneral, only the resulting third wave of difference frequency f₂−f₁ isof interest to this application. The higher frequencies only propagate ashort distance and tend to be absorbed in the non-linear materialitself. In some embodiments, the third wave or collimated beam has afrequency between approximately 15 kHz and approximately 120 kHz.However, a wider frequency range and higher frequencies are also withinthe scope of the present invention. In one embodiment, the frequencybandwidth of the third wave is determined by the two primary frequenciesf₁ and f₂ where one frequency (e.g., frequency f₁) is kept fixed and theother frequency (e.g., frequency f₂) is swept in time very rapidly(e.g., chirped). Hence, for example, by mixing a tone-burst of a fewcycles of high frequency (e.g. frequency f₁) with a frequency chirparound that frequency f₁ one can obtain a broadband signal. However, itis also possible to mix a variety of signals to create a desired timeresponse as well as a frequency response. For example, the compactparametric array source can be programmed to generate Gaussian pulsewith frequency range between approximately 15 kHz and approximately 120kHz by mixing two high frequency Gaussian pulses in theFluorinert-filled chamber. The resultant generated beam pulse atfrequency f₂−f₁ acts like an acoustic particle (analogous to phonon insolid state physics) traveling in the propagating medium. The sharppulse feature allows measurement with raw data without any kind ofsignal processing, such as cross-correlation and this speeds up themeasurement significantly. The experimental measurement system for theevaluation of this parametric array source for imaging features around aborehole casing is described in the next paragraph.

In one embodiment, the transducer 16 and mixing material 17 can bereplaced by an end-fire array of polyvinylidene difluoride (PVDF) filmacoustic source 30 shown in FIG. 1C. The end-fire of PVDF film acousticsource 30 comprises a plurality of spaced apart piezo-electric (PZT)layers (e.g., PVDF films) 32. PVDF provides some immediate benefits overpiezoceramics. PVDF has high mechanical damping and a complexpermittivity. Transducers constructed with PVDF can therefore have verybroad bandwidth, producing a pressure wave of short duration, thusoffering good spatial imaging resolution at lower operating (and henceminimally attenuating) center frequencies than piezoceramics.Additionally, the acoustic impedance (Z) of PVDF (MeasurementSpecialties, Norristown, Pa.) is approximately 2.7 MRayls relative tothe acoustic impedance of water which is equal to approximately 1.48MRayls. When using PZT layers, the non-linear mixing material or medium17 may be removed and substituted with any fluid that has goodtransmission properties at the desired operating frequency range (e.g.,between approximately 1 kHz and approximately 120 kHz) and low acousticabsorption. The PVDF films 32 can be mounted inside a housing 34 (e.g.,a cylinder). Although the end-fire array acoustic source 30 is describedherein as using PVDF films, as it can be appreciated, otherpiezo-electric films can be used. Although, the housing 34 is depictedin FIG. 1C as having a cylindrical shape with a circular base, thehousing 34 can have a cylindrical shape or configuration with anybase-shape (e.g., a polygonal base-shape). The acoustic source furtherincludes a non-linear medium filling between the piezo-electric layers(e.g., PVDF films). In one embodiment, the housing 34 is filled a mediumsuch as a fluid having an acoustic impedance substantially matching theacoustic impedance of the PVDF film 32. In one embodiment, the fluid canbe, for example, water as the acoustic impedance of the PVDF film 32substantially matches the acoustic impedance of water. In anotherembodiment, water can be replaced by Fluorinert (e.g., FC-43). Theimpedance mismatch between PVDF and fluorinert changes just slightly butthe sound speed in the liquid becomes significantly lower, that is 640m/s in FC-43 as compared to 1480 m/s for water. However, FluorinertFC-43 decomposes at elevated temperatures, over 390° F. The use ofFluorinert allows the size of the source to be decreased by almost onethird as compared to the size when using water because the acousticspeed in Fluorinert is lower. In one embodiment, the end-fire arraysource 30 further includes acoustic absorber material 31 disposed at afirst end of the housing 34 and a plate 33 disposed at a second end ofthe housing 34 opposite the first end. On one embodiment, the plate 33can be made into an acoustic lens to provide manipulation of theacoustic beam collimation or focusing, etc. The PVDF films provide avery broadband source of sound from 1 kHz to 100 MHz. In addition, inone embodiment, a lateral wall of the housing 34 can be layered withacoustic insulation 35 to prevent the acoustic waves generated by thePVDF films from reflecting from the lateral wall.

The end-fire array based on PVDF film acoustic source 30 is capable ofoutputting a more powerful acoustic wave (which can be, for example, ina form of cone or a collimated or parallel beam) than a conventionalparametric array using a single transducer. Each of the plurality ofpiezoelectric layers (e.g., PVDF films) is configured to generate anacoustic wave. The non-linear medium and the plurality of piezo-electriclayers have a matching impedance so as to enhance a transmission of theacoustic wave generated by each of plurality of layers through theremaining plurality of layers.

In one embodiment, an electrical generator such as electrical generator12 can be provided to electrically excite at least one piezo-electriclayer in the plurality of piezo-electric layers to generate an acousticwave pulse, as illustrated in FIG. 1D. For example, the electricalsignal generator 12 can be configured to electrically excite theplurality of piezo-electric (e.g., PVDF) films 32 to generate aplurality of acoustic wave pulses that are separated in time to form atrain of acoustic wave pulses. The electrical signal generator 12 can beconfigured to generate a wide variety of signal waveforms (tone bursts,frequency chirps, square waves, triangular waves, and any trigonometricwaveform shape etc.) in addition to a Gaussian pulse, and a cascade oftime delay generators. The time delay τ can be adjusted so that it isequal to the time for an acoustic pulse to propagate from one layer tothe other so that it arrives exactly at the time when the next layer isexcited. The time delay τ can be adjusted that it is substantially equalto a separation distance d between two consecutive PVDF films 32 dividedby the velocity “c” of sound in the medium between the consecutive PVDFfilms 32. Hence, if, for example, the first film 32A is excited at timet equal to zero to generated a first pulse, the second film 32B can beexcited at time t delayed by delay time τ to generate a second pulse,and the third film 32C can be excited at time t delayed by delay time 2τto generate a third pulse, etc. In this way, the first pulse generatedby the first film 32A arrives at the second film 32B at substantiallythe same time the second pulse is generated at the second layer 32B.Similarly, the first pulse and the second pulse arrive at the third film32C at substantially the same time the third pulse is generated at thethird layer 32C, etc. Each PVDF film 32 can be fed from these delaygenerators with the appropriate delay according to the position of thePVDF film 32 within the housing 34. Each PVDF film 32 can also beexcited by a delayed electrical signal whose amplitude can also beproperly adjusted and shaped. The purpose of this approach is to haveacoustic pulses from all previous layers or films to arrive at the lastlayer when the last layer is excited so that all the waves add up andproduce a strong pulse. If there are N layers then the signal emanatingfrom the last layer will be approximately N times the power generated byeach layer after subtracting off the loss of the signal in the layer andin the medium. Although it is simpler to have all layers positioned atequal intervals in space but that is not necessary. Indeed, the variouslayers can be positioned at any position and the interval between thelayers can be different. The time delay can be appropriately selected totake into account the separation between the various layers. A linearphased array approach with fixed frequencies can also be implemented byproperly varying the delay between the PVDF films 32.

In one embodiment, each PVDF film was excited by a 500 kHz tone burst.Frequencies from 50 kHz to 1 MHz may also be used if desired. There isno higher cut off frequency till almost 100 MHz and is only somewhatlimited mainly by the absorption of sound in the liquid that these filmsare immersed in. Experimental data is plotted in FIGS. 1E and 1F. FIG.1E shows the signal from all 4 PVDF transmitters when no electronicdelay is used. In this case, each signal arrived at the receiver basedon its distance from the receiver. FIG. 1F on the other hand shows whenappropriate time delay was used, all the signal arrived at the lasttransmitter at the same time. In this case, the detected signal by thereceiver now shows the large superimposed signal.

The efficiency of the end-fire acoustic source can be increased by usingPVDF films that are slightly curved instead of being stretched flat. Inone embodiment, each PVDF film can be provided with plastic cross madeof thin plastic wire (or a metal wire) attached to it so as to make thesurface of film slightly curved in a symmetrical manner. Each of thePVDF films has a thin layer of electrode on opposite sides whereelectrical connections are made for the excitation of the film. Thearray of films 32 is built into a wire frame and then inserted into thecylinder. The cables are brought out through an exit hole on theabsorber side of the cylinder.

For example, in operation, a first PVDF film 32A may be configured togenerate a first acoustic pulse, a second PVDF film 32B may beconfigured to generate a second acoustic pulse delayed relative to thefirst pulse, a third PVDF film 32C may be configured to generate a thirdacoustic pulse delayed relative to the second acoustic pulse, etc. Thethird PVDF film 32C can be configured to be transparent to the first andsecond acoustic pulses. The second PVDF film 32B can be configured to betransparent to the first acoustic pulse. The first, second and the thirdacoustic pulse are summed and, as a result, the PVDF film acousticsource 30 can output a signal comprising the first, the second and thethird acoustic pulses. This provides not only the ability to control thepower of the output acoustic beam but also the energy of the individualpulses and the delay between the various pulses. In this example, thePVDF film acoustic source 30 is described as having three PVDF films.However, any number of PVDF films can be used. In one embodiment, thehousing 34 is surrounded by an acoustic absorbing materials (not shown)to prevent an acoustic energy scattering to the side of the housing 34.

In another embodiment, the sound waves generated by each of the PVDFfilms 32 arrive at the front disc 33 at the same time and add up inpower. Each PVDF film is excited by an electrical pulse (Gaussianshaped) that has a signal bandwidth between about 15 kHz and about 120kHz. However, the PVDF can be excited to generate acoustic waves at anyfrequency range within the operating frequency of the films. In thisway, all the sound pulses generated by each element or PVDF film 32arrives at the front element 33 at the same time and sum up to produce apowerful signal that is close to N times the power output of eachelement after subtracting the small transmission loss in the liquid andthe film. The loss in the liquid is minimal at these frequencies.

In one embodiment, the collimated acoustic beam 18 can be steered in aparticular direction by an acoustic beam guide 20. In one embodiment,the acoustic source (transducers 16 and non-linear material 17 oracoustic source 30) and acoustic beam guide or steering device 20 aredisposed within a housing 22. The acoustic beam guide 20 can be anacoustic reflector or an acoustic lens, or a combination of both. Theacoustic reflector can be a material with different acoustic impedancefrom the surrounding medium in which the beam propagates. Onenon-limiting example of such an acoustic reflector is metal plate. Inone embodiment, the acoustic lens may be configured to focus thecollimated acoustic beam at a particular focal point and direction andcan have a concave shape. A Fresnel-type mirror arrangement can also beused for the acoustic beam guide. The acoustic beam guide 20 can berotated or tilted into a particular orientation by using one or moreactuators (not shown) coupled to the acoustic beam guide 20.Alternatively, in some embodiments, the acoustic beam guide 20 may notbe used, and the collimated beam 18 would propagate along the axis ofthe housing 22. For example, the housing 22 can be made of plastic orother suitable material. In one embodiment, the housing 22 can be in theform of a cylinder or pipe section with a circular base, as shown inFIGS. 1A and 1B. However, the housing 22 may have other configurationssuch as a cylinder with a polygonal base (e.g., square, rectangular,hexagonal, pentagonal, etc.). In one embodiment, the housing 22 may befilled with a liquid (e.g., water).

FIG. 14A depicts the acoustic measurement system disposed within aborehole, according to embodiment of the present invention. Thecollimated beam 18 can be steered to a particular direction toward anobject or target of interest such as a cement sheath or rock layersbehind casing 19A within a borehole 11 or object 19B (e.g., crack,fracture, void, etc.) within the rock formation 13 near the borehole 11,as shown in FIG. 14A. Inhomogeneities of formations, materials orstructures, such as object 19A, for example, will generate refraction orsurface wave propagation 21A which is scattered as acoustic wave 21B anddetected by receiver 24. Similarly, inhomogeneities within the rockformation 13 such as crack or fracture 19B creates a reflection orscattering of the acoustic beam 18 and the reflected acoustic wave 21Ccan then be detected by receiver 24. Acoustic beam 18 can generateelastic waves, e.g. refractions and surface propagation waves, travelingalong the boundaries with the rock formation 13 and boundaries betweenthe borehole and rock formation 13. The reflected, scattered waves orsurface waves and other types of waves are received by receiver 24.

FIG. 14B depicts the acoustic measurement system disposed within aborehole, according to another embodiment of the present invention. Inthis embodiment, the acoustic beam 18 output by the acoustic source 16,30 can be directed using steering device 20 downwardly generally in thedirection of axis 15 of borehole 11. In this case, the acoustic beam oracoustic wave 18 can be used to investigate rock formation 13 that hasnot been drilled and thus investigate ahead of the drill bit. This canbe performed, for example, during drilling operations. When the acousticbeam 18 is directed generally downwardly towards the rock formation 13,object(s) 19C (e.g., rock layers within the rock formation) reflect someof the acoustic beam energy 18 as acoustic wave 21D which can then bedetected by receiver 24. The location or distance of the object 19C fromthe acoustic source 16, 30 can then be determined based on the receivedacoustic wave 21D.

Similarly, amount of tilt of the layers 19C can also be determined basedon the inclination of the acoustic beam 18 (e.g., relative to boreholeaxis 15) and received acoustic wave 21D. FIG. 14C shows a situationwhere the layers 19C are tilted relative to the borehole 11 or boreholeaxis 15. In this case, the beam steering device 20 (e.g., a tiltableacoustic mirror or prism, etc.) which is disposed in front of the source30 can be used to direct the acoustic beam 18 in any direction includinga direction towards the layers 19C. If the steering device 20 ispositioned vertically, then it allows the sound beam from the source topass through without any significant amount of blockage. If the layers19C are tilted at an angle then the acoustic beam 18 will not bereflected by the layers 19C and as a result no reflected acoustic signalis detected by the receiver 24. If, on the other hand, the steeringdevice 20 is rotated or tilted such that the orientation of the acousticbeam 18 is substantially perpendicular or normal to layers 19C, anacoustic signal 21D can then be reflected from the tilted layers 21D andcan be detected by the receiver 24. The steering device can be orientedazimuthally in addition to inclination or elevation to provide acomplete picture of what lies ahead of the drilling bit.

As shown in FIG. 1B, the receiver 24 can also be provided within housing22. However, the receiver 24 can also be provided separate from thehousing 22 to allow independent movement of the receiver 24 and source16. The receiver 24 can be configured so as to receive the reflected,scattered, diffracted, etc. wave 21. In one embodiment, an acousticabsorber 23 can be disposed between the acoustic beam guide 20 and thereceiver 24, for example to prevent acoustic waves that may have notbeen reflected or otherwise directed by beam guide 20 from reaching thereceiver 24. In one embodiment, by placing the receiver 24 within thesame housing 22, the receiver 24 is able to receive the reflected orscattered wave 21 while the housing 22 is moved, i.e., the source 16 andthe receiver 24, and the housing 22, etc. are moved as a whole as asingle device 9 along the borehole 11 (as shown in FIG. 14). However, inanother embodiment, the acoustic source (e.g. the acoustic source 16with mixing material 17 or the acoustic source 30) and the receiver 24can be independently moved along the borehole 13. The reflected acousticwaves 21 are detected by receiver 24 and are converted into anelectrical signal which can be transmitted to processing electronics 26for analysis. The processing electronics 26 can include a computer withappropriate software for characterizing the rock formation or materialor structure surrounding the borehole, including producing 2D or 3Dimages of the formation or the material around the borehole 11.

In some embodiments, the entire device 9 including the transducers 16(or the end-fire acoustic source 30), the non-linear material 17, thesteering device 20, and receiver 24 can be moved up and down the lengthof the borehole 11 to image a particular formation near the borehole orinvestigate the structure of the borehole casing. However, in otherembodiments, the acoustic source (e.g., the acoustic source 16 withmixing material 17 or the acoustic source 30) and the steering device 20can also be moved independently from the receiver 24 (for example whilethe receiver is fixed). Moreover, the entire device 9 with or withoutthe receiver 24 can be rotated around the axis 15 of the borehole 11 toimage rock formations, structures, materials, etc. in any azimuthaldirection around the borehole 11.

FIG. 2A is a schematic representation of the receiver 24, according toan embodiment of the present invention. The receiver 24 comprises aplurality of receiver elements 40. The receiver elements 40 can be anarray of PVDF films. In one embodiment, the array can be produced from asingle PVDF sheet with properly depositing electrodes on both sides ofthe film (or etching out a previously metallized electrode over theentire surface) and leaving a gap between neighboring elements. Each ofthese electrodes then behaves as a piezoelectric receiver element. Atypical array element size can be approximately 1 cm×1 cm but it can bealmost any size depending on the needed resolution of the experiment. Inone embodiment, electrical lines can be laid down on the film or thePVDF sheet for electrical connections. The entire sheet with electrodescan then be covered with a very thin sheet of material (e.g., Mylar) forprotection and electrical shorts. Therefore, a linear array can bewrapped around, as shown in FIG. 2B, as a circular configuration madearound an acoustically absorbing material (e.g., foam) to create acircular array that covers 360 degrees. The receiver elements 40 aremounted on surface 42S of an acoustic absorbing material (e.g., acousticabsorbing foam, sponge or various types of silicon rubber) 42. FIG. 2Cdepicts a schematic representation of the receiver 24, according toanother embodiment of the present invention. In this embodiment, a largesheet of PVDF film can be used to create an array of 2-dimensionalarrangement (i.e., a matrix arrangement) of transducer receiver elements40. The array of transducers 40 can then be wrapped around a cylindricalconfiguration to create an array that can provide complete 360 degreecoverage around the axis of the cylindrical configuration, in additionto vertical coverage along the axis of the cylindrical configuration. Inthis way, the receiver array does not need to be physically rotatedazimuthally in the borehole. In this case, a different vertical seriesor rows of PVDF layers or a one-dimensional array of PVDF films withinthe two-dimensional array of PVDF film can be electronically selected todetect acoustic signals. Typically, in operation, all PVDF elements canbe scanned first at a given location to determine the direction fromwhich any signal is coming and then the appropriate vertical arrays canbe used to track this signal. Signal multiplexor electronics can be usedto carry out this kind of electronic scanning and the detected signalcan be subsequently amplified and digitized. As shown in FIGS. 2B and2C, the acoustic absorbing material 42 has a cylindrical configurationwith a circular base. However, as it can be appreciated, the absorbingmaterial 42 can have any desired configuration such as a cylindricalconfiguration with a polygonal base or elliptical base, or other shape.By mounting the receiver elements 40 on the surface 42S of the absorbingmaterial 42, the receiver elements 40 receive acoustic signals from thefront side of the receiver elements 40 and not from the back side of thereceiver elements 40.

FIG. 3 is schematic diagram of a laboratory measurement system orexperimental set up to test the measurement system when deployed in aborehole environment, according to an embodiment of the presentinvention. In the experimental set up, the housing 22 including theacoustic source (e.g., the acoustic source 16 with mixing material 17 orthe acoustic source 30), the beam steering device 20 and the receiver 24are positioned within an axial borehole 11A in a barrel (e.g., a cementbarrel) 29 that simulates the borehole 11 with a cement casing. Theacoustic measurement system 9 includes acoustic source (e.g., theacoustic source 16 with mixing material 17 or the acoustic source 30),mirror system 20 and receiver 24. In one embodiment, acoustic source(e.g., the acoustic source 16 with mixing material 17 or the acousticsource 30), non-linear medium 17, mirror system 20 and receiver 24 aredisposed inside housing 22. In one embodiment, the receiver 24 isconfigured such that it only receives acoustic signals from the front.The receiver 24 is shielded from other signals such as back signals(i.e., signals that are incident on the back of the receiver 24 areabsorbed by absorber 42). In one embodiment, the receiver 24 isconfigured to move with acoustic source (the acoustic source 16 withmixing material 17 or the acoustic source 30). In another embodiment,the receiver 24 can be moved independently of the acoustic source (theacoustic source 16 with mixing material 17 or the acoustic source 30).In order to test the efficacy of this measurement system, a groove 25 isprovided at an outer periphery or outer surface of the barrel 29 (e.g.,concrete or cement barrel), as will be explained further in detail inthe following paragraphs.

In one embodiment, the acoustic source (the acoustic source 16 withmixing material 17 or the acoustic source 30) and the receiver 24 areconfigured such that the beam direction from the acoustic source (theacoustic source 16 with mixing material 17 or the acoustic source 30),i.e., acoustic beam 18, and the received signal 21 lie on the sameplane. In one embodiment, both the acoustic source (the acoustic source16 with mixing material 17 or the acoustic source 30) and receiver 24are rotated azimuthally from 0 to 360 degree. However, in anotherembodiment, only the mirror 20 is rotated while the source (the acousticsource 16 with mixing material 17 or the acoustic source 30) and thereceiver 24 are fixed. Indeed, by providing receiver 24 with acylindrical configuration where receiver elements 40 are disposed on thesurface of the cylindrical configuration, the receiver 24 is able todetect an acoustic signal at angle from 0 to 360 deg. without having tomove or rotate the receiver 24. Similarly, the acoustic source (theacoustic source 16 with mixing material 17 or the acoustic source 30)need not be reoriented to be able to scan a desired field azimuthalangle. The azimuthal field angle can be scanned by simply rotating thesteering device (e.g., mirror 20). The acoustic beam emitted by theacoustic source (the acoustic source 16 with mixing material 17 or theacoustic source 30) is reflected by the beam steering device (e.g.,mirror) 20 and directed as acoustic beam 18 towards inner wall of cementbarrel 29. The acoustic beam 18 interacts with the material of thebarrel 29, the material outside barrel 29, the interface between thehousing 22 and the barrel 29, etc., and generates reflections,refractions or surface waves, or any combinations of thereof. In a firstscenario, the acoustic beam, after being reflected by the mirror 20, mayundergo a reflection by the material of the barrel (e.g., cement barrel)29 or the rock formation, or both. The reflected acoustic signal maythen be detected by receiver 24. This scenario is generally referred toas a reflective mode. In a second scenario, the acoustic signal, afterbeing reflected by the mirror 20, may be refracted by the material ofthe barrel (e.g., cement barrel) 29 at the interface between the cementbarrel 29 and the rock formation. The refracted acoustic signal may thenbe detected by receiver 24. This scenario is generally referred to as arefractive mode. In yet a third scenario, the acoustic signal, afterbeing reflected by mirror 20 may generate surface waves at the interfacebetween a surface of the borehole and the cement in the barrel 29 (orrock formation in a field deployment) or at interface boundaries withinthe cement (or rock formation). The surface waves will emit returningacoustic signals that can be detected by the receiver 24. This scenariois generally referred to as a surface wave mode.

FIG. 4A illustrates a characteristic of the beam pulse signal emitted bythe parametric acoustic source disposed within the borehole in alaboratory experimental set up, according to an embodiment of thepresent invention. The acoustic beam signal pattern 45 on the exteriorsurface of the barrel 29 as a function of time (time domain) is measuredusing a laser Doppler vibrometer. The waveform of the signal 45 is shownin FIG. 4A. FIG. 4B is a fast Fourier transform (FFT) of the acousticbeam signal 45 to obtain the signal in the frequency domain. Thefrequency bandwidth of the signal 45 can be extracted from the FFTshowing a broad frequency bandwidth between about 15 kHz and about 120kHz.

The unique characteristics of the acoustic source (the acoustic source16 with mixing material 17 or the acoustic source 30) can be combinedwith various receiver elements or modules 40 into a measurement systemto perform azimuthal borehole sonic measurements, three-dimensional (3D)reflection imaging from a borehole, 3D refraction imaging, 3D fracturedetection, 3D mapping of permeability, and 3D mapping of channelsbetween the cement barrel and rock formation.

Because the high directivity of the beam pulse, many of the deficienciesof the existing borehole acoustic measurement systems cited above can beminimized. As discussed below, the system has good azimuthal resolutionas well as inclination direction control. In one embodiment, theazimuthal angular resolution is between about 5 deg. and about 15 deg.,for example 10 deg. This new capability enables the extension ofborehole acoustic measurement to full 3D measurement (the 3^(rd)dimension being the azimuthal angle).

FIG. 5 depicts data collected as a function of propagation time,distance between receiver elements and acoustic beam source andazimuthal angle in an experiment using the experimental setup shown inFIG. 3, according to an embodiment of the present invention. In thisexperiment the beam source is directed at the rock formation at oneazimuthal angle and one inclination angle, and the linear receiver 24with receiver elements 40 is oriented to detect the returning signal inthe same azimuthal angles as the source beam, as shown in FIG. 6. Theentire assembly of source, mirror and receiver are rotated azimuthallyin incremental steps of 10 degrees and the returning acoustic signalsdata are recorded for all receiver elements for each azimuthalincrement. FIG. 5 shows five panels labeled as panel 1 to panel 5 (P1,P2, P3, P4 and P5). Each panel corresponds to data displayed for oneazimuth measurement (i.e., azimuthal angle). Each 10 deg. azimuthalangle (i.e., 0 deg., 10 deg., 20 deg., etc.) corresponds to a differentpanel (P1 through P5). The y-coordinate in each panel represents thearrival time of the signal detected at the various receiver elements 40.The x-coordinate in each panel corresponds to the distance from verticalreceiver element to the source. The gray scale of the displaycorresponds to the amplitude of the received acoustic signal. Withineach panel are shown a plurality of data points 58. Each of these points58 corresponds to a signal detected by one of the plurality of thereceiver elements 40 of receiver 24. In this example, receiver 24 isprovided with 12 receiver elements 40. Therefore, 12 data points aredetected by the receiver 24, each point corresponding to a signaldetected by one of the 12 receiver elements 40. Each of the 12 datapoints has a different arrival time corresponding to the arrival of thesignal to each of the 12 receiver elements 40. As shown in FIG. 5, thefirst linear signal arrival 50 corresponds to P-wave compressionrefraction wave commonly measured in sonic log. The second and thirdlinear signal arrivals 52 and 54 correspond to surface waves such asRayleigh, Stoneley or Lamb waves. Signal arrivals due to reflection fromcement/air interface at barrel perimeter are shown at 56.

FIG. 7 depicts reflection data obtained in an experiment similar to thedata shown in FIG. 5A but after performing signal processing to filterout the linear arrivals. There are 36 panels P1-P36 and each panelcorresponds to an azimuthal angle and the 36 panels range from 0 to 180degrees. For example, panel P1 corresponds to azimuthal angle of 0 deg.The y-coordinate represents the arrival time at the receiver 24. They-coordinate in each panel represents the arrival time of the signaldetected at the various receiver elements 40. The x-coordinate in eachpanel corresponds to the vertical distance from receiver element to thesource. The gray scale of the display corresponds to the amplitude ofthe received acoustic signal. Within each panel, i.e., within eachazimuthal angle range, hyperbola-like curves 59 can be seen. Each curve59 corresponds to data of a signal detected by one single receiverelement 40 in the receiver 24. The series of wave patterns 60 and 62correspond to a reflection from a perimeter or outer periphery of thecement barrel 29 while the wave pattern 64 corresponds to a reflectionfrom a surface of the groove 25 (at an interface of the cement and air).As it can be noted, the waves reflected from the surface of the groove25 arrive to the receiver 24 earlier than waves reflected from thecylindrical surface of the barrel 29. Furthermore, the position of thegroove 25 can be ascertained by using the azimuthal measurement methodand system described herein. The present method achieves excellentazimuthal resolution which allows detecting defects within a structuresuch within a casing within a borehole or at an interface of theborehole and the rock formation, etc. For example, as it can be noted inFIG. 7, the groove 25 can be located at specific azimuthal angles orwithin an azimuthal angular range allowing a determination of a positionor location of a structure, such as a structural defect, a fracture, orthe like.

FIG. 8 depicts a different data display of the same experiment with adifferent sorting. There are 12 panels (from P1 to P12) in FIG. 8, Eachpanel (P1, P2, . . . , P12) corresponds to data of signals detected byone of the 12 receiver elements 40 in receiver 24. Within each panel(e.g., panel P1) the x-coordinate represents the azimuthal angle (in therange from 0 deg. to 360 deg.). The y-coordinate represents arrival timeat each of the 12 receiver elements 40 of receiver 24. The gray scale ofthe display corresponds to the amplitude of the received acousticsignal. As can be seen in FIG. 8, the reflection from the groove 25 isdetected by some detector elements 40 (for example, at panels P1 throughP4) and not by other detector elements (for example, at panels P9through P12). In addition, it can be noted that, for panel P1 forexample, the groove 25 is clearly seen in the middle of the panel whichcorrespond to an azimuthal angle around 90 deg. The reason for detectingthe groove 25 with specific receiver elements 40 (panels P1 through P4)and not by other receiver elements 40 (panels P9 through P12) is due tothe fact that the acoustic beam 18 has a specific angular elevationspread and thus is reflected selectively to specific detector elements40. Hence, detector elements 40 (corresponding to panels P9 throughP12)) that are outside of the scattered, reflected, diffracted acousticwave beam from the groove 25 are not able to detect the reflected,diffracted, scattered beam from groove 25. However, as it can beappreciated, if the receiver 24 is moved vertically, other receiverelements 40 within the receiver 24 can then detect the signal reflected,diffracted or scattered by the groove 25. In this case, the groove maythen be seen in panels P6 through P10 if the inclination of the groovechanges, for example. Hence, the present measurement system is not onlycapable to resolve a position of a structure in azimuthal angle but alsoin elevation angle as well.

Furthermore, the elevation information can be utilized to determine anorientation of the structure (e.g., groove 25). For example, in thelaboratory experiment described in the above paragraphs, the groove 25is parallel to the axis of the borehole in the cement barrel 29.However, the grove 25 can also be positioned oblique, i.e., at an anglerelative to the borehole axis, in which case, the angular elevationinformation which depends on the orientation of the groove 25 can bedifferent. Indeed, depending on the angular orientation of the structure(e.g. groove 25) relative to the borehole axis, the reflected,diffracted beam by the groove 25 can be directed preferentially tospecific receiver elements 40. As a result, the groove 25 can be seen inthe plotted data or image at different panels (e.g., at panels P7 andP8). By determining in which panels the groove 25 is detected, it ispossible to infer the angular inclination of the groove 25.

FIG. 9 depicts another experiment in which the orientation of thereceiver 24 is fixed (i.e., the receiver is not rotated) and the mirroris rotated azimuthally between 0 and 360 degrees at an increment of 20degrees. 19 panels are displayed with each panel corresponds to signaldata recorded with one the azimuthal angle from 0 deg. to 360 deg.azimuthal angle at 20 degree increment. The y-coordinate representsarrival time at the receiver elements 40 of receiver 24. Thex-coordinate in each panel corresponds to the vertical distance betweenthe receiver element and the source. The gray scale of the displaycorresponds to the amplitude of the received acoustic signal. The dataclearly shows excellent azimuth resolution with the maximum energy ofthe linear arrivals occurring when the beam orientation and receiverreception orientation are aligned. This shows that the propagation paththat is rather narrow in extent and does not spread too muchazimuthally.

FIGS. 10A and 10B depict an experimental acoustic setup, according toanother embodiment of the present invention. FIG. 10A is a longitudinalschematic view of the experimental setup and FIG. 10B is a top view ofthe experimental setup. The experimental setup includes is similar inmany aspects to the experimental set up shown schematically in FIG. 3.The cement barrel 22 is lined with a steel axially arranged inner casing100. A pipe or tube 102 is embedded within the cement barrel 22. Agroove 25 is also cut or carved on an exterior surface of the cementbarrel 22. A detachment foil 104 (e.g., Aluminum foil) is also providedwithin the housing 22. In this embodiment, the detachment foil 104 isdisposed in contact with inner casing 100. Within the casing 100 of thecement barrel 22 are disposed the acoustic source 16, 30, the non-linearmaterial 17, the mirror 20 and the receiver 24. As shown in FIG. 10B,axes are drawn to indicate azimuthal angular orientation (theorientation of the two axes is arbitrary). The groove 25 is located atazimuthal angle between about 230 deg. and about 280 deg. The pipe ortube 102 is located at an azimuthal angle between about 80 deg. andabout 100 deg. The detachment foil (e.g., aluminum foil) is located atan azimuthal angle between about 140 deg. and about 190 deg.

FIGS. 11A-11C show plots of the measured data for various azimuthalorientations or angles, respectively, at about 320 deg., at about 90deg. and at about 165 deg., according to an embodiment of the presentinvention. The azimuthal orientation or angle of about 320 deg, (FIG.11A) corresponds to the orientation of the acoustic beam in a regionwhere there is no inclusion behind the inner casing 100, i.e., there isonly the cement barrel wall. The azimuthal orientation or angle of about90 deg. (FIG. 11B) corresponds to the orientation of the acoustic beamin a region where the tube (e.g., plastic pipe) 102 is included. Theazimuthal orientation or angle of about 165 deg. (FIG. 11C) correspondsto the orientation of the acoustic beam in a region where the detachmentfoil (e.g., aluminum foil) 104 is provided. In this plot, they-coordinate corresponds to the time it takes for the acoustic wave tobe received by receiver 24, the x-coordinate in each panel correspondsto the vertical distance of receiver element from the source. Thevarious curves in each plot correspond to the acoustic signals receivedby the various receiver elements 40 in receiver 24. In this example,there is provided 12 receiver elements 40 in receiver 24. However, anynumber of receiver elements can be used. The curve closest to thex-coordinate corresponds to the signal detected by the first receiverelement and the curve farthest to the x-coordinate corresponds to thesignal detected by the 12^(th) receiver element. The first receiverelement is the receiver element that is closest to the acoustic source16, 30 and the 12^(th) receiver element is the receiver element that isfarthest from the acoustic source 16, 30.

As shown in FIG. 11A, with no inclusion behind the inner casing 100, thesurface waves decay with distance along the borehole, i.e. decay fromthe first receiver element to the 12^(th) receiver element. As shown inFIG. 11C, with the delamination or detachment foil 104 behind the casing100, the surface wave amplitude is larger and decays more slowly asexpected because the steel pipe is not dampened by the contact with thecement (i.e., the aluminum foils carries the acoustic waves fartheralong the borehole). As shown in FIG. 11B, at azimuth anglescorresponding to the pipe 102, the surface wave amplitude is larger andalso decays more slowly. In addition to the surface wave, a fast lineararrival just behind the P-wave first arrival is recorded indicatingadditional wave mode traveling along the wall of the pipe 102. Thismeasurement data clearly show that azimuthal information of rockformation behind the steel casing can be gleaned from linear arrivalsusing a borehole acoustic measurement system.

In addition to the ability of changing the azimuthal orientation of theacoustic source beam by changing the azimuthal angular direction of themirror 20, the inclination of mirror 20 can also be changed to send theacoustic source beam along any vertical direction. This allows theacoustic source beam to be injected at different inclinations andazimuthal directions to probe for reflection boundaries, refractionboundaries and fractures of different orientations in the rockformation. The data can be subsequently analyzed using variousconventional methods. Analysis of refraction arrivals along withazimuthal resolution can provide for 3D imaging of velocity byrefraction analysis. This can provide better characterization of nearborehole alteration and characterization of the skin of reservoirs.

In one embodiment, the measurement data are collected using broadbandbeam pulse. In this way, information with broad frequency bandwidth canbe collected relatively quickly. Indeed, in this case, there is no needto sweep the frequency by chirping. Furthermore, in one embodiment, theuse of multiple acoustic sources to cover the entire the bandwidth, forexample, between about 15 kHz and about 120 kHz, may not be needed. Theacoustic beam pulse with broad bandwidth, for example between about 15kHz and about 120 kHz, can provide measurements that can yieldinformation on cement bonding between the cement and the rock formationin a borehole.

The present measurement system can be used for evaluating a cementcasing or steel casing in a borehole. A simulation of guided wavepropagation through the steel casing when a sound beam pulse interactswith the steel casing is performed under certain geometrical conditions.In this simulation, a 25 mm thick layer of cement is used between thesteel casing and Berea sandstone. The Berea is considered infinite inextent. It is also assumed that the borehole is filled with water andthere is energy sink along the axis of the borehole. The simulationswere carried out using the DISPERSE software package from the ImperialCollege, UK.

FIG. 12A-12C show plots of the acoustic simulation in the frequencyrange of 20-120 kHz for various conditions. The data in these plots arecaptured in the instance where the borehole is filled with water. Eachdata set is generated under different condition but in each case thereceiver is at a distance of 12 inches from the excitation point on thesteel casing in the axial direction. These data are showing thepropagation characteristics of a sound pulse (frequency chirp) of 100microsecond duration with a frequency span of 20-120 kHz and with aGaussian envelope. The graphs on the left side show the amplitude of thereceived acoustic signal as a function of time and the graphs of theright side show the fast Fourier transform of the acoustic signal to thefrequency domain where the amplitude is plotted as a function of thefrequency. FIG. 12A is a plot of the data captured withwater-steel-concrete-air, where there is an air gap between the concreteand the Berea sandstone. FIG. 12B is a plot of the data withwater-steel-concrete-water-Berea, where there a water gap (e.g., a 1 mmgap) between the concrete and Berea. FIG. 12C is a plot of the datacaptured with water-steel-concrete-Berea, where everything is theinterfaces between the water, steel, concrete and Berea are in physicalcontact.

The plots depicted in FIGS. 12A-12C show significant differences amongthem in terms of the characteristics of the signal. When the concrete isin good contact with the Berea sandstone, the energy of the wavesthrough the steel dissipates into the Berea and the observed amplitudeis rather low (as shown in FIG. 12C). When there is a detachment or gapbetween concrete and Berea, the signal level is higher (as shown inFIGS. 12A and 12B).

The second set of plots on the right which represent the amplitude ofthe signal vs. the frequency shows the frequency content of the receivedsignal. Higher frequencies are damped out when the concrete and theBerea are in good contact (as shown in FIG. 12C). In addition, as can benoted in FIG. 12B, the presence of water between the concrete and Bereaconfines the energy to earlier times and the frequency content is alsonarrowed. As shown in FIG. 12A, when the concrete is in good contactwith Berea, the signal spreads out in time with the main arrival delayedsignificantly. The differences between the various scenarios can bereadily seen in these plots. The simulated data shows that the describedmeasurement method or system can be used effectively for cementevaluation around a borehole casing.

FIGS. 12A-12C above shows the frequency content of the propagated signalunder various conditions of borehole casing integrity in reference tothe concrete and the Berea rock formation behind it. Therefore, onecannot see in these figures which frequencies are propagating at whatstrength at different times. Another way to view the informationpresented in FIGS. 12A-12C can be based on a joint time-frequencyanalysis of the data using a short-time Fourier transform (STFT)approach. This provides the frequency content of the signal as afunction of time and thus allows one to see the frequencies that areprominent at certain times during the propagation. Hence, the STFTanalysis of the data enhances the information provided by FIGS. 12A-12Cand introduces a powerful analysis approach.

FIGS. 13A-13C show the original simulated frequency chirp propagationdata along with the time-frequency analysis of the same data. The plotson the right represent the 3D time-frequency information for each of thesituations discussed above with reference to FIGS. 12A-12C,respectively. The x-axis corresponds to the time, the y-axis correspondsto the frequency, and the z-axis or vertical axis corresponds to theamplitude. In FIG. 13A, as shown in the 3D plot, where the concrete isdetached from the rest of the system and does not see the rockformation, the energy in the waves propagate through at three differentvelocities and this gives rise to the three peaks at 0.1 second timeinterval. It also noted that the wave also arrives relatively quickly,after 0.1 second. In FIG. 13B, as shown in the 3D plot, the situation isthat there is a 1 mm gap filled with water between the cement and therock formation. The propagation characteristics of the acoustic wave arecompletely different from the propagation characteristics of theacoustic wave shown in FIG. 13A. Indeed, all the energy seems to bebunched together and propagates relatively quickly through the casingand the cement and the propagation is not influenced by the rockformation as if the two parts are isolated. FIG. 13C depicts thesituation where all the layers are tightly coupled (steel casing, cementand the rock formation). As shown in the 3D plot in FIG. 13C, thepresence of the rock formation has a strong loading influence on thewave propagation and the wave propagation is delayed significantly andthe main energy peak arrives with a delay of almost 0.5 second. Thesethree examples show how the various detachments or coupling between thelayers can be detected by this type of analysis and measurements.

In addition, by providing azimuthal resolution in borehole acousticmeasurements, rock characterization can be improved and thus improveproduction engineering systems. Furthermore, by proving azimuthalresolution in borehole acoustic measurement, the integrity of theborehole can be evaluated and thus improve the overall drilling safety.In addition, azimuthal resolution in borehole measurements can allowmeasure a stress surrounding the borehole and as a result improveborehole completion methodology.

Furthermore, the borehole acoustic measurement system and methoddescribed can also be used for imaging the rock formation, indeed, thepresent measurement system and method can fill a measurement gap betweenconventional sonic tools that investigate less than a foot(approximately 33 cm) from the borehole with relatively a good verticalresolution and conventional long range sonic image tools such asborehole acoustic reflection survey (BARS), from Schlumbergercorporation, which investigate rock formation at tens of feet from theborehole but with lower vertical resolution and limited azimuthalresolution. For example the present acoustic measurement method andsystem may be utilized in various applications including:

1. 3D imaging of reservoir layers, stratigraphy, fractures, faults, vugs(up to few feet such as 10 feet from the borehole) with full azimuthresolution.

2. Measurement of compressional velocity Vp and shear velocity Vs of therock formation with full azimuth determination.

3. 3D analysis of geo-mechanical properties around boreholes fromanalysis of refraction waves and Lamb waves to improve characterizationof the invasion zone and any borehole damage.

4. 3D imaging of velocity of rock formation near the borehole usingrefraction analysis.

5. 3D mapping of fractures from reflections of linear arrivals

6. 3D mapping of permeability and production skin of reservoirs.

7. Focusing the acoustic beam with a phase-code Gaussian pulses in thelower frequency range, e.g., between about 10 kHz and about 30 kHz fordeeper penetration into the rock formation while discriminating againstbackground noise.

For example, in one embodiment, measurement of the compressional and/orshear velocity of the rock formation in the vicinity of the borehole ata plurality of azimuthal angles using the above described measurementsystem can provide valuable information on the stress around theborehole hence allowing determining or predicting potential fractureposition and/or fracture propagation with the rock formation in thevicinity of the borehole. It is known that formations having relativelylarger velocity variations are either relatively less consolidated, orthe stress in the formation is large. In both situations, this mayprovide an indication as to the likelihood of borehole collapsing. Theacoustic measurement system described in the above paragraphs canprovide information on the velocities as a function of azimuthal angleand/or elevation angle within the rock formation around the borehole.Using the velocity as a function of azimuthal angle and or elevationangle can in turn provide the azimuthal and/or inclination angle ofvarious stress areas and/or fractures, faults, etc., and thus canultimately provide information on the anisotropy of the earth stressfield around the borehole. In addition, the position of a fracture orfault can be mapped in 3 dimensions (3D mapping) using the data acquiredas a function of azimuthal and elevation angle.

The above described measurement system and method can also be used inmapping fluid permeability of subsurface formations such as sub-surfacespenetrated by a borehole including permeability due to fractures in therock formation. For example, this can be performed by measuringvelocities (compression velocity or shear velocity or surface waves orany combination of the velocities cited) at various points within therock formation around the borehole. Based on the measured velocity, thepermeability can be extracted using various known models.

In one embodiment, the method or methods described above can beimplemented as a series of instructions which can be executed by acomputer. As it can be appreciated, the term “computer” is used hereinto encompass any type of computing system or device including a personalcomputer (e.g., a desktop computer, a laptop computer, or any otherhandheld computing device), or a mainframe computer (e.g., an IBMmainframe), or a supercomputer (e.g., a CRAY computer), or a pluralityof networked computers in a distributed computing environment.

For example, the method(s) may be implemented as a software programapplication which can be stored in a computer readable medium such ashard disks, CDROMs, optical disks, DVDs, magnetic optical disks, RAMs,EPROMs, EEPROMs, magnetic or optical cards, flash cards (e.g., a USBflash card), PCMCIA memory cards, smart cards, or other media.

Alternatively, a portion or the whole software program product can bedownloaded from a remote computer or server via a network such as theinternet, an ATM network, a wide area network (WAN) or a local areanetwork.

Alternatively, instead or in addition to implementing the method ascomputer program product(s) (e.g., as software products) embodied in acomputer, the method can be implemented as hardware in which for examplean application specific integrated circuit (ASIC) can be designed toimplement the method.

FIG. 15 is a schematic diagram representing a computer system 130 forimplementing the methods, according to an embodiment of the presentinvention. As shown in FIG. 15, computer system 130 comprises aprocessor (e.g., one or more processors) 132 and a memory 134 incommunication with the processor 132. The computer system 130 mayfurther include an input device 136 for inputting data (such askeyboard, a mouse or the like) and an output device 138 such as adisplay device for displaying results of the computation. The computersystem 130 may be configured to control various modules including acontrol module 140 to control the signal generator 12, a control module142 to control the steering of the mirror 20, and acquisitionelectronics 26 for acquiring the measurement data. The measurement datacan be stored in a storage device (e.g., a flash drive) for lattervisualization or processing, etc.

In one embodiment, there is provided a system for investigatingstructure near a borehole. The system includes an acoustic sourceconfigured to generate an acoustic wave and to direct the acoustic waveat one or more azimuthal angles towards a desired location in a vicinityof a borehole. The system further includes one or more receiversconfigured to receive an acoustic signal, the acoustic signaloriginating from a reflection or a refraction of the acoustic wave by amaterial at the desired location. The system also includes a processorconfigured to perform data processing on the received signal to analyzethe received acoustic signal to characterize features of the materialaround the borehole.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, it is to be understood thatthe present invention contemplates that, to the extent possible, one ormore features of any embodiment can be combined with one or morefeatures of any other embodiment.

Furthermore, since numerous modifications and changes will readily occurto those of skill in the art, it is not desired to limit the inventionto the exact construction and operation described herein. Accordingly,all suitable modifications and equivalents should be considered asfalling within the spirit and scope of the invention.

What is claimed is:
 1. An acoustic detector, comprising: a cylindricalsupport member; and a plurality of receiver elements disposed on asurface of the cylindrical support member, the plurality of receiverelements being configured to detect acoustic waves in a plurality ofazimuthal angular directions.
 2. The acoustic detector according toclaim 1, wherein the plurality of receiver elements are configured asone-dimensional or a two-dimensional array of receiver elements disposedon a surface of a cylindrical support member.
 3. The acoustic detectoraccording to claim 1, the plurality of receiver elements comprise apiezo-electric film.
 4. The acoustic detector according to claim 3,wherein the piezo-electric film comprises a PVDF film.
 5. The acousticdetector according to claim 3, further comprising a plurality ofelectrodes disposed on both sides of the piezo-electric film, theelectrodes being spaced apart from each other so as to leave a gapbetween neighboring receiver elements.
 6. The acoustic detectoraccording to claim 1, wherein the plurality of electrodes on thepiezo-electric film are produced by etching, by vacuum depositing, or bycoating a conductor material on both sides of the piezo-electric film.7. The acoustic detector according to claim 1, wherein the plurality ofreceiver elements are configured as a one-dimensional array or atwo-dimensional array of receiver elements.
 8. The acoustic detectoraccording to claim 7, wherein the one-dimensional array or thetwo-dimensional array of receivers is wrapped around the surface of thecylindrical support member so as to cover a 360 deg. detection angle. 9.The acoustic detector according to claim 8, wherein a row ofone-dimensional array of receivers within the two-dimensional array ofreceivers are electronically selectable to detect the acoustic waves.10. The acoustic detector according to claim 1, wherein the cylindricalsubstrate comprises an acoustic absorbing material.
 11. The acousticdetector according to claim 10, wherein the acoustic absorbing materialcomprises an acoustic absorbing foam or silicon rubber.
 12. The acousticdetector according to claim 1, wherein a size of the receiver elementsis selected to achieve a desired azimuthal angular resolution of theacoustic wave between approximately 5 deg. and approximately 15 deg.